In general, bearing systems serve to absorb and transmit forces, both axial and radial, between components moving relative to one another. This means that bearings are necessary for virtually all orbital, rotary or swivelling motions. Bearings are thus an indispensable element of all machines and assemblies and in particular in internal combustion engines.
A study of the most important bearing points (main bearing, connecting rod bearing, small end bearing bush, camshaft bush etc.) in modern internal combustion engines provides a good overview of the multiplicity of frequently contradictory properties which the bearing material must exhibit. Depending on the type of bearing and engine involved, account may have to be taken not only of different structural conditions, but also of wholly different types of load (gas forces, inertia forces, sliding speeds). Over the course of time, composite materials exhibiting a multilayer structure have proven particularly well suited to meeting the resultant varied requirements (high fatigue strength, good wear resistance, low susceptibility to scoring, high corrosion resistance, high cavitation resistance etc) . Thus, two- and three-layer composite materials today constitute part of the prior art. In such an instance, a steel backing member lends a plain bearing the necessary mechanical strength and provides the correct interference fit in the housing. The bearing metal, applied by roll-bonding, strip cast-coating or sintering, provides the above-mentioned properties, an additional overlay, generally applied by electroplating and having an intermediate layer acting as a diffusion barrier, completing the composite system.
Development trends in relation to the diesel engines of the future show two main marginal requirements: minimization of fuel consumption and reduction in pollutant emissions.
In comparison to conventional diesel engines, this is achieved by means of direct injection, i.e. increasing combustion pressure, and by turbocharging. Hence, in new generations of engines, extremely high demands are made particularly of bearing materials. Clearly, the trend is towards materials exhibiting ever higher load-carrying capacities, which has already resulted, in the case of connecting rod bearings, in new composite multilayer materials. Thus, in this field the sputter bearing (the overlay consists of a sliding layer applied by means of cathodic sputtering) has become a commercial fixture. With this type of bearing, extremely high loads may be carried without difficulty.
However, in other areas too, such as small end bearings, the demands made of bearing materials have also risen, to such an extent that they can only be met by the steel/lead-bronze-based bush materials traditionally used in this field by increasing the small end diameter, which results in reduced specific loading. However, this development is a move in the wrong direction, since it results in an increase in the oscillating masses and thus in the structural height of the engine, which runs counter to the general desire for weight reduction. A further problem in using lead-bronze alloys is their inadequate corrosion resistance.
From this it is clear that, in the sphere of bush materials for example, future market requirements will be fulfillable only by the development of a new composite material system. Of this the following technical and economic demands will be made: the composite material must be able to withstand loads of the highest order, it must exhibit a high level of corrosion resistance at temperatures of up to 200.degree. C. in an aggressive environment (material highly stressed by oil additives, combustion residues in the oil and heavy contamination of the oil as a result of relatively long intervals being maintenance checks) and it must be cost-effective to produce.
In EP 0 681 114, a composite multilayer material is described which consists of steel with a plain bearing material of a copper-zinc wrought alloy, as used for bearing bushes or thrust washers. Production of this composite material is effected by means of roll-bonding. Heat treatment subsequent to the roll-bonding increases the bond strength between the steel and the bearing metal as a consequence of diffusion processes.
In contrast to the method claimed in this application, production of this composite system entails a roll-bonding process. The roll pressure results in mechanical adhesion of the two materials by interlinking of the surfaces. Although subsequent homogenization strengthens this bond, it does not result in a positive connection or even a metallurgical bond, as is the case with coating by casting, i.e. contact between a liquid phase and a solid phase.
Furthermore, it should be noted that the process described in EP 0 681 114 is also more expensive to carry out, by comparison, than is coating by casting, since before the composite material is produced by roll-bonding, the CuZn31Si strip has to be produced by its own casting process. The composite material is produced by roll-bonding only in a further stage. However, with coating of steel by casting the composite material may be produced in a single stage.
DE-OS 25 48 941 describes a method of producing strand-form, metallic items, in which method several layers of the same material are applied. Consequently, several coating vessels are provided. The layer which is respectively formed on the strip at the coating point is continuously drawn off and cooled. To this end, appropriate cooling devices are provided beneath the strip.
A method of strip-casting lead-bronze is known from DE-PS 10 63 343, in which the steel strip is heated to a temperature of approximately 1100.degree. C., to prevent distortion of the strip. The strip is previously formed into a U-shaped profile with bent edges. After cast-coating and cooling of the strip, which, however, is not described at all, the strip is milled to the desired thickness and then coiled.
A method of producing a steel composite cast material is known from DE 44 37 565 A1. This method is not a continuous strip cast-coating process, but rather a vertical or centrifugal casting process, in which bearing shells which have already been shaped are coated. This copper-based bearing alloy contains nickel and silicon in a given ratio, such that the brittle iron silicide phases are suppressed in the bond zone. The sliding element to be coated is preheated, the temperature to which it is preheated being selected as a function of the thickness of the steel. This method is suitable only for large bearings and thus expensive components. This known method cannot be used for mass production, as is necessary in the case of bearings of smaller size with steel thicknesses below 10 mm.